If you leave it to your insurance, your case will be assigned to an arbitrator. Problem is that the arbitrator will go the easy route and follow police report, thus confirming that it wasn't the truck's part that hit you.

It's appaling that the 18 wheeler chose that route, but he had it planned since he gave you wrong insurance card.

I would go to that location one more time and see if I can find the part. This is a long shot and you have to be very careful also not to get hit by another car in the highway. Talk to your insurance and see what happens if they decide that the part didn't fall off the truck. If they say you are going to be liable, then fight this off. Hire a lawyer; the only case you can make is that the truck driver gave you some insurance paper-which he would never have done if he didn't think a part fell off his truck.

Good luck.